Mountain bike component compatibility pitfalls

Component Compatibility Pitfalls

Ever wondered about building a custom mountain bike?  Well, there’s a lot to it, and I figured as I build up my own new dream bike, this Ibis Mojo HD4 wonder machine, I could explain some of the potential pitfalls to avoid when choosing components for a modern trail bike.

My Mojo HD4 build!  All links go to Jenson USA, a sponsor of mine:

Also check out the new Ibis Ripmo here:

First up, today’s complete bikes are the best they’ve ever been.  That said, maybe you need something different- I build my bikes specifically to my style, location, and needs.  Everyone is different, so if you’re going custom, here’s a way to keep yourself in check!

Step 1: pick your frame- and if you’re doing a custom build, get something nice.  Typically, frames will be sold with a rear shock already fitted, which is a great thing, and the tune to that shock will often have some degree of customization.  And it’s also good because it means you won’t have to mess with finding the right size reducers that will adapt the shock to the frame’s mounting points.

Step 2: most modern mountain bike forks will be a 1.5 tapered, and fork travel is generally within ~20mm of the frame’s travel.  This bike is a 153mm travel frame and I’m using a standard 160mm Fox 36 RC2 fork.  Pitfall!!!  If you happen to have a straight steerer fork, there are simple adapters that will allow them to fit a 1.5 Tapered frame, and you can fit a 1.5 Tapered fork into a 1.5 straight head tube frame, but you cannot put a 1.5 tapered fork into a 1 1/8” straight steerer frame.  All forks will need their steerer tubes cut down- or the opposite, simply long enough.  New forks are almost always long enough- re-using an old fork, or buying used, is where a short steerer can be a problem.

Step 3: Choose your headset, and utilize the fit guide from your frame manufacturer to be sure you have the right configuration for your bike.  PITFALL!!!  Some frame brands do mix and match headset sizes depending on the various models, so this is a key detail to be 100% sure on.

Step 4: select stem and handlebar.  1.5 tapered forks use a 1 1/8” stem, and most modern trail bikes are designed around a 40-70mm stem.  Standard 31.8mm handlebars and stems work great, and some people like the larger 35mm design.  PITFAL!  Be sure that you have the same size for the stem and the bar.

Step 5: select your wheels.  For best results, use the wheel size that is intended for your frame.   Pitfall!  Various hub sizes exist.  Most modern frames are “Boost 148mm” in the rear, though some are “Super Duper Boost 157” and some older designs are still 142×12.  Traditional frames with open rear drop outs measure 135mm wide by 10mm diameter.  Specify the rear hub width as needed for your frame.  Front hubs will generally be either Boost 15mm x 110mm, 15mm by 100mm, or old school 9 x 100mm.  Many older, or more free ride set ups, are a 20x110mm front axle, and that is indeed different than a 20x110mm Boost front hub.  Triple check that you are getting compatible parts here.

Second pitfall! Not all hubs can be adapted to work with all available widths.  Double check the sizes you’ll need- every now and then I find myself stuck with a boost fork and an old school 15x100mm hub.  Final pitfall, be sure you’re ordering the free hub you’ll need: either a Shimano 9/10/11 speed, a SRAM XD driver, or a Shimano 12 speed mountain.

Step 6: decide which drivetrain you want to use.  Be sure your free hub matches, and that the front chainring isn’t too large for your frame.  Drivetrain choices are generally Shimano or SRAM.  Be sure the shifter, derailleur, chain, cassette, chainring, and crankset are all designed to be used with the same type of system- either 9, 10, 11, 12 speed.  Pitfall! Avoid mixing and matching components intended for differing drivetrain types, such as a 10 speed chain on an 11 speed bike.  Modern trail bikes don’t use front derailleurs, which simplifies things quite a bit.  Most trail bikes will use either a 68 or 73mm bottom bracket, and those are cross-compatible with the included BB spacers simply taking up the difference.  Press Fit BBs are fairly standard, and I’ll leave it up to you to let me know in the comments if the whole press fit idea is a pitfall within itself.  Final Pitfall, BBs aren’t always included with cranks, so double check if you need to buy one separately.

While I think these are a necessity, I am in the minority here- If you’re running a chain guide, be sure you have the ISCG adapter for your frame.  Some brands require these to be purchased separately.

Step 7: Modern bikes use dropper seatposts. PITFALL! Get that diameter correct! While the difference between 31.6 and 30.9 might not sound like much, it’s significant.  Be sure you order the diameter specific to your frame.  Also, pay attention to whether you need an internal or external routing.  Posts usually do include levers.

PITFALL: The problems that you can have, though, are either that seat tubes just don’t allow for much insertion depth, or that bikes with big wheels can allow the rear wheel to hit the saddle at bottom out.  Just because you want to run a 170mm dropper on your size small 29er doesn’t mean that set up will actually clear at bottom out.  Read the recommended dropper specs from the website of your frame supplier.

Step 8: Hang the brakes.  Pitfall 1: Rotors come in two mounting types; center lock and 6 bolt.  Be sure to order the type that works for your hubs.  Then, final pitfall- mounting tabs!  A 180mm front/rear set up is often easiest.  Some forks come set up for 180mm rotors natively, others come for 160mm.  Some frames use a 160mm post mount, others a 180mm, and some are still using an IS mount.  Check with your manufacturers to see what the native brake mounts are, and buy your adapters accordingly.

After all these PITFALLS, are you ready for a breath of fresh air?  Pedals, grips, saddles, and tires are all refreshingly standardized- for now.

And finally, if you’re really confused, then work with a good retailer, such as Jenson USA.  They sponsor me, and they are all about helping folks order the correct parts.  Give them a call.  If you have any questions for me about why I built my HD this way, please let me know if the comments below


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1 comment

  1. I will have to add all this techy stuff to my DVD when I get around to it…what a fountain of information..ever think of working for a bike company…better yet building your own…JKW specials…Happy New Year.

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